Breast cancer represents 32 percent of all cancers in women. Yet, Oriental women consuming a traditional diet high in soy products have a lower incidence of breast cancer. We have demonstrated that genistein, a component of soy, when injected in prepubertal female rats suppressed the development of chemically-induced mammary cancer. Prepubertal genistein treatment enhanced mammary gland differentiation and resulted in a lower rate of cell proliferation. Now, we propose to investigate dietary genistein exposure for mammary cancer prevention (dose response studies). We will determine the doses to use from studies on gland differentiation, using mammary whole mounts, and cell proliferation, using bromodeoxyuridine immunohistochemistry (IHC). We hypothesize that early exposure to genistein alters gene and protein expression of specific steroid and growth factor pathways to result in maturational effects that render the glands/cells more differentiated and less susceptible for cancer. To investigate mechanisms of action, we propose to measure protein expression (by IHC and western blot) of the estrogen-, progesterone-, and EGF-receptors, and TGF-alpha, EGF, and TCF beta, all modulated by estrogen. We will also investigate the potential of genistein in the diet to alter the reproductive/endocrine system of these animals. This application is a competing continuation of one of two R01 applications submitted as interactive proposals. The title of the IRPG application is "Genistein: Bioavailability, Mechanisms, Cancer Prevention". The overall goals of our interactive research proposals are to investigate the potential of genistein, a component of soy, to protect against mammary cancer, and to determine the pharmacokinetics and mechanisms of action of genistein in vivo.